Electrified railway-systems with filtered booster-transformers



Aug. 2, 1955 L, 1. HlBBARD 2,714,671

ELECTRIFIED RAILWAY-SYSTEMS WITH FILTERED BOOSTER-TRANSFORMERS FiledFeb.

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ATTORNEY United States Patent Gfce ma rf er? p i wit i i attentati Aug.55

ELECTRFIED RAi'LWAY-SYSTEMS WITH FILTERED BSTER-TRANSFRMERS AppiicationFebruary 26, 1953, Serial N o. 333,981 l2 Claims. (Cl. 307-90) Myinvention relates to improvements in the prevention of inductiveinterference in telephone lines, telegraphlines, or othercommunications-lines which parailel a single-phase railway-system. Thenovel feature of the invention is the use of a low-pass filter, or atleast a fundamental-frequency filter, connected in shunt circuitrelation with a booster-transformer or transformers, for forcing thevoice-frequency harmonics of the electrically propelled vehicle toreturn through a fixed return-circuit which is so located that itsinductive eifects materially cancel out the inductive ef'r'ects of thecorresponding voicefrequency currents which are carried by thetrolley-wire or power-supply line. This fixed return-circuit may beeither the track-rails or an elevated pilot-wire which is adjacent toboth the troliey-wire and the telephoneline.

Heretofore, in booster-transformer systems in which the secondarywindings of the booster transformers were connected in series with thetrack-rails, and in which no lter was used, it was necessary to forcethe fundamental-frequency currents, as well as all of the harmonics, toow through the rails. This has not been a very practical or desirablesystem, because of the necessity for limiting the maximum possiblesecondary voltage of the booster transformer to a value which will besafe for man and beast, even under short-circuit conditions. lf thisrequirement is met, even for full-load conditions alone, it entails theuse of many booster-transformers which are closely spaced along theright-of-way; and if the limited-voltage requirement is met when thepowercircuit is short-circuited, this requirement has meant that thespacing between successive booster-transformers has had to be very smallindeed.

ln previous interference-eliminating systems using booster-transformersfor forcing all of the return-circuit currents, both fundamental andharmonic, to flow through a pilot-wire suspended near the trolley-wire,it has been necessary to use rather large pilot-wires, in order to beable to carry the total load-currents, and in order also to have animpedance which is suiciently low as compared to that of thetraer-rails.

in accordance with my present invention, the main fundamental-frequencyload-current still returns through the normal rail-and-ground path,spreading out through the ground in the normal manner. Thisfundamentalfrequency current does not usually produce any objectionableinductive interference in adjacent telephone-circuits, and if it shouldcause any objectionable induced noise, in telephone-lines which areunusually sensitive, the trouble can be easily eliminated by using aband-pass filter in series with the telephone line, so as to admit theflow of only the most necessary voice-transmitting frequencies, such asa band which eliminates all the frequencies below 250, 300, or 400cycles, or the like, as is well known in telephone practice. Such avoice-frequency filter, in the telephone line, could also eliminate allextremely high-frequency harmonics, above, say, 3,000 cycles, or

-A, each of which may include a power above any other desired upperlimit; or no upper limit at all could be imposed by the voice-frequencyfilter.

By the simple expedient of bypassing the fundamentalfrequency currentfrom the booster-transformer or transformers, l cause this transformeror these transformers to have a substantially Zero fundamemal-frequencysecondary-voltage, thus immediately mitigating one of the big drawbacksto such booster-transformers, which is the fundamental-frequencysecondary-voltage. At the same time, l reduce the size of thebooster-transformers, and l reduce the necessary size of the suspendedpilot or returnwire, up near the trolley, if such a wire is used,besides achieving other advantages, some of which will be pointed out inthe course of the subsequent description.

in the usual case, none of the return-circuit frequencies causesobjectional telephone-interference, below a certain minimum limit, whichmay be of the order of 250, 300 or 400 cycles; and hence, instead ofusing a filter which is tuned only to the fundamental frequency, forbypassing the booster-transformers, l can frequently (if not always) usea low-pass filter which substantially filters out all such lowfrequencies from the booster-transformer or transformers. lf avoice-pass filter is used in the adjacent telephone-circuit, then thelow-pass lter for the boostertransformers can materially exclude atleast all frequencies which are substantially below the minimumfrequency of the voice-pass filter, so that this low-frequency band iseliminated from the booster-transformer or transformers.

My invention is applicable not only to railway-systems usingrectifier-powered vehicles of the type in which the single-phasepower-current is rectified, in the vehicle, and delivered todirect-current traction-motors, but the invention is also applicable tosingle-phase railroad-systems which utilize single-phasecommutator-motors for traction. All of such railway-systems produce acertain spectrum of harmonics, differing somewhat in distribution andmagnitude, but always involving a more or less serious problem ofinductive interference in neighboring telephone circuits. ln most suchsingle-phase railwaysystems, it is feasible to design the electricalequipment which is carried by the vehicles so that no objectionabletelephone-interference is encountered in most interurban running intoinductive-interference troubles only 1n some urban areas, or in specialtrack-sections where the inductive-interference problem is particularlysevere. My present invention makes it possible to inexpensively takecare of these special inductive-interference areas, so that the entireequipment need not be designed to meet the most aggravatedinductive-interference problems which exist anywhere on therailway-system.

With the foregoing and other objects in view, my invention consists inthe circuits, systems, combinations, apparatus, parts, and methods ofdesign and application, as hereinabove described, and as illustrated inthe accompanying drawing, wherein the ve figures show different types ofillustrative forms of embodiment of my invention, in the form ofsimplified diagrammatic representations of different circuits andapparatus.

In Fig. l, for example, I show an electrified railwaysystem, includingtracks or track-rails l, and an insulatedly supported bare or uncoveredsingle-phase trolleywire or power-supply line 2, and one or moresubstations transformer 3 having a secondary winding 4 which isconnected between the track-rails 1 and the trolley-wire 2. Thetrolleywire 2 is disposed in a position suitable for supplyingsingle-phase power to one or more electrically propelled vehicles Vwhich ride on the tracks l. One such vehicle is diagrammaticallyrepresented as having a pantograph 5, which is intended to berepresentative of any including a third-rail shoe.

current-collecting device,

3 The vehicle-load is diagrammatically represented by the primarywinding of a transformer 6, and the vehicle is further diagrammaticallyrepresented as having only one wheel 7, although it will, in general,have at least four wheels and usually many more than four wheels. ltwill be understood that the bare power-supply line 2 need not be anoverhead trolley-wire or conductor; it may, for example, be a thirdrail. In the particular form of my invention which is shown in Fig. l,however, the bare power-supply line 2 is actually a overheadtrolley-wire, at least in the portion of the system which is shown inFig. l.

Since my invention is designed to prevent or mitigate inductiveinterference, it is necessarily used in a system in which there is aninductively coupled telephone-line or other communications-line orcircuit 8, which is adjacent to the power-supply line 2, or to theright-of-way of the railway-system. The inductive interference arisesfrom the fact that the tracks 1 serve jointly as a vehiclesupportingmeans and also, return means for the currents which are required by thevehicle. If all of the currents, harmonics as well as fundamental,returned through the track-rails, there would practically never be anyinductive-interference problem. lt is well known, however, that sincethe track-rails 1 are grounded, the ground-return currents spread outinto the ground, leaving the rails and spreading out overa rather widearea, and then coming back into the rails again at the substation A. Itis these spread-out currents, in the ground, which cause a mismatchingof the inductive-interference effects of the trolley-currents and theground-return currents, thus inducing interference-producing currents inthe adjacent telephone-wires 8. More strictly speaking, it is only thespread-out harmonic ground-currents, in the voice-frequency range, causeinductive interference in the adjacent telephoneline 8. l

My invention relates broadly to any means which limits this spreadingout of voice-frequency harmonic ground-currents, while permitting thefundamentalfrequency currents, or a low-frequency band including thefundamental, to spread out into the ground in the usual way, thusreturning from the vehicle to the substation through a combination ofrail and ground.

In the particular form of embodiment of my inveni tion which is shown inFig. l, I use a special overhead auxiliary or pilot-wire or conductor 11to provide a iixed return-circuit for the harmonics. At one or morepoints along the railway right-of-way, I equip both the trolleywire 2and the pilot-wire and 11' respectively. The trolley-wire gap 2 isshunted by the primary winding 13 of an iron-core boostertransformer 15.This booster-transformer has a secondary winding 17 having the samenumber of turns as the primary winding and insulated from the primarywinding, this secondary winding 17 being connected across the pilot-wiregap 11. At a plurality of other points along the right-o-way, the pilotwire 11 is connected to the tracks 1 by means of a plurality of spacedtrackconnections 18. The idea of distance is conveyed, in the drawing,by means of dotted-line sections in accordance with a usual convention.

In accordance with my present invention, at least one of the windings ofeach booster-transformer 15 is shunted by a lter F. In Fig. l, thisiilter F is shown as a seriesresonant circuit, comprising a capacitor Cand an inductance coil L, tuned approximately to the fundamentalfrequency of the power-supply line 2, which may, for example, be 25cycles or 60 cycles, or even some lower frequency such as l5 cycles.

This filter F may be connected, either across the secondary winding 1'7of its associated booster-transformer, as indicated by the circuit 21 inFig. l, or across the primary winding 13 of its booster-transformer, asindicated by the circuit 22 in Fig. l. The secondary-circuit conatleastin part, as a groundwhich 11 with an insulated gap, 2'

nection 21 has a possible advantage, in that it more perfectly keeps thefundamental-frequency current out oithe pilot-wire 11, by reason of thefact that it practically short-circuits the secondary terminals 23, 24of the booster-transformer 15. On the other hand, the primarycircuitconnection 22 has the advantage of substantially completely bypassingthe fundamental-frequency current around the booster-transformer 15, sothat there is no fundametnal-frequency terminal-voltage in thisboostertransformer, either across the primary terminals 25 and 26, oracross the secondary terminals 23 and 24. The voice-frequency harmoniccurrents, however, pass through the booster-transformer, whether it isprovided with the primary lter-circuit 22 or the secondaryfilter-circuit 21,

` or both, so that the booster-transformer develops whatever voltagesare necessary to cause the same voice-frequency harmonic currents toflow, in equal magnitudes and opposite directions, in the trolley-wire 2and in the pilot-wire 11.

It will be noted that both of the filter-circuits 21 and 22, in Fig. l,whether connected across the secondary winding 17 or the primary winding13, have substantially the same effect with respect to the filtered-outcurrent or frequency. That is, they cause the booster-transformer tohave substantially no exciting-current, and hence substantially nomagnetic ux and substantially no terminal Voltage, across either itsprimary 13 or its secondary 17, with respect to the filtered-outcurrent. This results in a considerable saving in the amount oftransformer-iron which is needed for the transformer-core of thebooster-transformer. When the filter-circuit is connected across theprimary winding 13 of the booster-transformer, as shown at 22 in Fig. l,the heavy fundamental-frequency load-currents are kept substantiallyentirely out of the booster-transformer, so that neither the primarywinding 13 nor the secondary winding 17 needs to be designed to carrythese heavy fundamental-frequency currents, thus saving materially inthe cost and size of the booster-transformer. It may be noted, also,that the exclusion of the primary currents from the transformer, as bymeans of the filtercircuit 22, avoids any possibility ofsaturation-diiculties, due to either full-load currents or short-circuitconditions. But perhaps of even greater importance, my iilters F in Fig,l avoid any possibility of excessive induced fundamental-frequencyvoltages in the booster-transformers.

lt will be understood that the ilter F, in Fig. l, may be connected,either across the secondary winding, as shown at 21 in Fig. l, or acrossthe primary winding, as shown at 22 in Fig. l, or somebooster-transformers can be filtered in one Way, while others areiiltered in the other way, or both primary and secondaryfiltering-circuits may be used on each booster-transformer.

Since pilot-wire booster-transformer interference-preventing means, butwithout my filtering circuits, have been known for some time, havingbeen extensively used in Sweden, for example, and since such systemshave been fully explained in the technical press, a detailed explanationof the operation of my invention, as shown in Fig. l, is not believed tobe necessary, except to note that my addition of the ltering means makesthe pilotwire 11 effective to prevent or reduce inductive interference,only with respect to the harmonics, and not with respect to thefundamental-frequency currents. The fundamental-frequency currents arepermitted to return in the normal or usual rail-and-ground return-path.To illustrate this operation, two different kinds of currentindicatingarrows have been added, in Fig. l, the fullline arrows representing thefundamental or filtered currents, while the dotted-stem arrows indicatethe noniltered harmonics. The auxiliary pilot-wire 11 carries only theharmonics, as represented by the dotted-stem arrows. At thetrack-connection 18 which is closest to the substation A, the harmoniccurrents which are carried by the auxiliary return-circuit wire 11return to the track-rails 1, and thus return to the primary circuit ofthe power-transformer 3, and thence they go out through the trolley-wire2, as is indicated in Fig. 1.

Fig. 2 shows two modifications of the circuit which is shown in Fig. 1.The first modification is in the addition of a telephone-line voice-passfilter VP, in series with the telephone-circuit 8. This voice-passfilter VP substantially excludes all frequencies below the minimumfrequencies necessary for satisfactory voice-reproduction. Such minimumfrequencies may be 250 cycles, 300 cycles or 400 cycles, or the like, ashas been previously explained. The voice-pass lter VP may permit thetransmission of all frequencies higher than this lower limit, or it may,if desired, exclude certain unnecessary extremely high frequencies, suchas all frequencies higher than anything in the order of 2,800 cycles or4,000 cycles, more or less.

The second modification which is introduced in Fig. 2 is the replacementof the fundamental-frequency resonant-circuit filter F of Fig. 1, with alow-pass filter LP, which passes all frequencies which are substantiallywhich is passed by the voiceis believed to be necessary.

In Fig. 3, I show a modification of my invention in which, instead ofusing a harmonic-frequency pilot-wire return-path, I force theaudible-frequency harmonic-currents to be confined substantially to thetrack-rails 1, without spreading out through the surrounding ground toany material extent, while at the same time permitting thefundamental-frequency or lowest-frequency returncurrents to flow intheir normal rail-and-ground paths. To this end, I connect the secondarycircuits of the booster-transformers across insulated gaps 1 in thetrack-rails 1. A fundamental-frequency or low-pass filter is connectedacross either the primary circuit or the secondary circuit of thebooster-transformers 1S, as indicated at F1 and F2, respectively, aswill be understood from the discussions given in connection with Figs. land 2.

transformers 15, frequency voltages there may be required in order tokeep the harmonic-frequency current-flow, in the rails, equal andopposite to the harmonic-frequency currentflow in the trolley-wire 2,thereby forcing these harmonic currents to flow through thetrack-rails 1. this result, however, without forcing themental-frequency track-rails 1 for their return-circuit path, therebygreatly reducing the burden on the booster-transformers 1S, and alsoavoiding the excessive-voltage difficulties resulting from the heavyfundamental-frequency currents, as previously noted.

Fig. 4 shows an advantageous variation over the circuit shown in Fig. lor Fig. 2, in which I add an additional filtered eterminal-based orstation-based boostertransformer or transformers 15', and an additionalterminal track-connection 18', at each of the various substations alongthe right-of-way, such as the substation A. This substation isrepresented as being located at some intermediate terminal or locationin the railway-system, not at the extreme terminal or end of the system,so that it may deliver current to either the right or the left, to thetrolley-wire 2. In Fig. 4, a tliree-Winding l-to-l ratio station-basedbooster-transformer 15 is shown by way of illustration, having a primarywinding 13, and two secondary windings 171 and 172, all having the samenumber of turns. In Fig. 4, the booster-primary 13 is connected inseries with the power-transformer circuit 4, so that whatever current issupplied to the trolleywire 2 by that substation will be carried by thewinding 13 of the booster-transformer 15. The boostersecondaries 171 and172 are connected between the trackthe ligure. It will be noted thatacross the primary winding 13 of the booster-transformer 15.

It is believed that the operation of the apparatus shown in Fig. 4 willbe apparent from the foregoing explanations and from the various arrowswhich are shown in the station-based boostertransformer 15 forces theharmonic currents to leave the pilot-Wire 11 right at the substation,through the track-connection 18', instead of returning through the nextspaced track-connection 18 along the right-of-Way, as in Figs. l and 2.

Fig. 5 is a modification of the Fig. 4 circuit, in which the spacedboosters 15, along the right-of-way, are omitted,

has a sufcient voltampere rating to torce the pilot-wire harmoniccurrents to return through the pilot-wire 11. In this form of embodimentof my invention, there will be some spreading-out of the pilot-wireharmonic currents, between the vehicle V and the station A, particularlyif a very small pilot-wire 11 is used. Thus, these harmonic currentsrails 1 and the ground, by reason of the spaced trackconnections 18along the riglit-of-way; but in many cases the operation of the Fig. 5circuit will be good enough, and the additional cost of the Fig. 4circuit will not be required. By way of illustration, tlie Fig. 5circuit uses secondary-connected filters F2, instead of theprimaryconnected filter F1 of Fig. 4.

While I have shown several bodiment of my invention, I wish it to beunderstood that I am not limited to these particular forms, as variouscombinations and substitutions of the illustrative equipments shown inthe various figures may be made, or still other changes may be made byway of the addition or omission of refinements, and the substitution ofequivalents, without departing from the essential features of myinvention. l desire, therefore, that the appended claims shall beaccorded the broadest construction consistent with their langauge.

I claim as my invention:

l. In an electrified railway-system including tracks which serve jointlyas vehicle-supporting means and also, at least in part, as ground-returnmeans, an insulatedly supported bare single-phase power-supply linedisposed in a position suitable for supplying single-phase powerpropelled vehicles riding on said tracks, and an inductively coupledcommunicationsline adjacent to said power-supply line; the combinationincluding one or more booster-transformers, each iiaving a primarywinding and a secondary winding electrically insulated from each other,a means for electrically connecting the primary winding of saidbooster-transformer in series-circuit relation with respect to saidpowerfor providing a fixed return-circuit illustrative forms ofemcurrents carried by said fixed return-circuit materially cancel outthe inductive by said power-supply line being an overheadtrolleyconductor, and said fixed return-circuit being an auxiliaryoverhead conductor adjacent to both the trolley-conductor and thetelephone-line, in combination with means for connecting said auxiliaryoverhead conductor to the tracks at each of a plurality of spacedpoints.

4. The invention as defined in claim 1, characterized by saidfilter-means including a series-resonant circuit tuned approximately tothe fundamental frequency of the power-supply line and connected inshunt relation to the booster-transformer.

5. The invention as defined in claim 4, characterized by such aseries-resonant circuit being connected in shunt relation to thesecondary winding of the boostertransformer.

6. The invention as defined in claim 4, characterized by such aseries-resonant circuit being connected in shunt relation to the primarywinding of the booster-transformer.

7. The invention as defined in claim 1, characterized by saidfilter-means being operative to materially exclude the lower harmonics,as well as the fundamental, from at least one of the windings of thebooster-transformer.

8. The invention as defined in claim 7, characterized by saidfilter-means including a means operating substantially like alow-frequency band-pass filter connected in shunt realtion to thebooster-transformer.

9. The invention as defined in claim 8, characterized by such alow-frequency band-pass filter-means being connected in shunt relationto the secondary winding of the booster-transformer.

l0. The invention as defined in claim 8, characterized by such alow-frequency band-pass filter-means being connected in shunt relationto the primary winding of the booster-transformer.

l1. The invention as defined in claim 1, in combination with a band-passfilter-means, serially included in the communications-line, formaterially excluding, from said communications-line, at least allfrequencies below the minimum frequency which is substantially necessaryfor the communications-line.

12. The invention as defined in claim 11, characterized by thelter-means of the booster-transformer being operative to materiallyexclude, from at least one of the windings of the booster-transformer,at least all frequencies substantially below said minimum frequencywhich is substantially necessary for the communicationsline.

References Cited in the file of this patent UNITED STATES PATENTS1,010,982 Taylor Dec. 5, 1911 1,295,886 Fortescue Mar. 4, 1919 1,651,449Evans Dec. 6, 1927 1,770,705 Lucas July 15, 1930 1,803,454 Afel May 5,1931 2,469,138 Van Gilder May 3, 1949 2,554,248 Hibbard May 22, 1951FOREIGN PATENTS 471,380 France July 8, 1914

